Paraxylene is a basic raw material for producing polyester. Presently, paraxylene is mainly obtained by disproportionation, isomerization, adsorption separation or cryogenic separation of a raw material containing toluene, C9 aromatics and mixed xylene from naphtha catalytic reforming products. By this method, the target product paraxylene ratio in mixed C8 aromatics is only about 20% due to the reason that the reaction of producing paraxylene is controlled by thermodynamics. Its equipment is huge with high operating costs due to the large recycle material quantity, and an expensive adsorption separation system is necessary because a high purity paraxylene product cannot be obtained by common distillation.
With the increasing shortage of oil resource, many research institutes in domestic and overseas began to work actively on development of new techniques for producing aromatics, such as producing aromatics from methanol, or from liquefied gas etc. Methanol to aromatics technology provided a new pathway to prepare aromatics from coal or natural gas, wherein aromatics can be directly obtained by aromatization reaction of methanol or dimethyl ether on a composite catalyst containing metal and zeolite.
In 1977, Mobil's Chang et al (Journal of Catalysis, 1977, 47, 249) reported a method to prepare aromatics and other hydrocarbons on ZSM-5 zeolite catalysts from methanol or other oxygenates. It showed that alkanes and olefins can be converted to aromatics under certain conditions under the effect of metal components. Therefore, it has become a main research direction that the ZSM-5 zeolite catalysts are modified by metal component to produce more aromatics by conversion of methanol.
At present, the modification of ZSM-5 zeolite catalysts are mainly focused on using Zn and Ga, while the use of other metals such as Ag, Cu have also been reported. Ono et al (J Chem. Soc, Faraday Trans. 1, 1988, 84 (4), 1091; Microporous Materials, 1995, 4, 379) investigated the catalytic performance of methanol to aromatics (MTA) process on the ZSM-5 zeolite catalysts which were modified by Zn and Ag using ion exchange method. It revealed that the yield of aromatics in products increased to about 67.4% (C %) after ZSM-5 zeolite was modified by Zn, and increased to about 80% after ZSM-5 zeolite was modified by Ag.
CN 101244969 discloses a fluidized bed apparatus, containing an aromatization reactor of C1-C2 hydrocarbon or methanol and a catalyst regeneration system. By adjusting catalyst coking state in aromatization reactor, a continuous efficient conversion of C1-C2 hydrocarbons or methanol to aromatics with high selectivity is realized.
CN 1880288 discloses a process with high total aromatics selectivity, and which can be operated flexibly. As the main product, aromatics are obtained from methanol conversion on modified ZSM-5 zeolite catalysts.
A ZSM-5 zeolite catalyst loaded by Zn and Mn is disclosed in U.S. Pat. No. 4,615,995. The catalyst is used for methanol conversion to olefin and aromatics. In products, the ratio of low carbon olefins to aromatics can be controlled by adjusting the content of Zn and Mn loaded in catalyst.
It is an effective way to expand the sources of aromatics by the aromatization of liquefied petroleum gas (LPG) from oil refinery byproducts. In process of LPG aromatization to aromatics, the high quality gasoline and aromatics such as benzene, toluene and xylene (BTX) can be obtained simultaneously. U.S. Pat. No. 4,642,402 discloses a process of light hydrocarbons conversion to aromatics, which is developed by UOP, converting C3 and C4 to BTX aromatics by aromatization reaction on a ZSM-5 zeolite catalyst modified by Ga, wherein the aromatics yield is about 60%.
A HZSM-5 zeolite catalyst modified by gallium, zinc, platinum is disclosed in CN 1023633C, using for aromatization reaction of low carbon alkane. The catalyst is applicable for aromatization of C5-C8 alkane, especially olefins to the mixed aromatics containing benzene, toluene and xylene. The total aromatics yield is about 50 wt %.
CN 1660724A discloses a fluidized bed process which is used for aromatization of liquefied gas to benzene, toluene and xylene on DLC-2 catalyst, and the total yield of benzene, toluene and xylene is about 65 wt %.
The said conversion processes of methanol or liquefied gas to aromatics, which all utilize one single raw material to obtain the target mix aromatics: benzne, toluene and xylene (BTX), possess low market value, and thus poor economic profit.